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Benefit Plans Obligations and Funded Status (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2023
Change in benefit obligation      
Actuarial Gain (Loss) from pre-Medicare Trend Rate Assumptions $ 30    
Actuarial Gain (Loss) from Changes in Mortality Assumptions 5    
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Discount rate   5.00% 6.05%
Amount of defined benefit plan actuarial gain (loss) from updated capita claims costs and retiree contributions (18)    
Amount of defined benefit plan actuarial gain (loss) from change in mortality assumption   $ (46)  
Amount of defined benefit plan actuarial gain (loss) from change in demograhic and plan experience 37    
Actuarial Gain (Loss) from Changes in Withdrawal Assumption Rates 2    
Actuarial Gain (Loss) from Observed Plan Experience $ 2    
Percent Funded Status 10000.00%    
Pension Benefits      
Change in benefit obligation      
Benefit obligation $ 11,002 10,099  
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation 10,301 11,002  
Service cost 32 29 $ 32
Defined Benefit Plan, Interest Cost 526 579 568
Plan participants' contributions 3 3  
Change in Plan Assets due to Collections 0 0  
Collections [1] 0 0  
Actuarial loss (gain) 457 1,059  
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement and Curtailment (1) 0  
Defined Benefit Plan, Cost of Providing Special and Contractual Termination Benefits 0 0 $ 0
Net transfers from variable fund/401(k) plan 2 5  
Expenses paid (7) (6)  
Benefits paid 799 766  
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Plan Assets 8,594 8,673  
Change in plan assets      
Fair value of net plan assets 8,673 8,129  
Actual return on plan assets 412 1,004  
Employer contributions 310 304  
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Funded status $ (1,707) $ (2,329)  
Discount rate 5.47% 4.95% 5.95%
Amount of defined benefit plan actuarial gain (loss) from discount rate change $ (514) $ (981)  
Amount of defined benefit plan actuarial gain (loss) from change in mortality assumption (39)    
Amount of defined benefit plan actuarial gain (loss) from change in demograhic and plan experience 18    
Other Post-retirement Benefits      
Change in benefit obligation      
Benefit obligation 291 353  
Postconfirmation, Other Postretirement Obligations   347  
Service cost 11 11 $ 10
Defined Benefit Plan, Interest Cost 17 21 18
Plan participants' contributions 0 0  
Change in Plan Assets due to Collections 12 12  
Collections [1] 12 12  
Actuarial loss (gain) 67 4  
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement and Curtailment 0 0  
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits 1 0  
Defined Benefit Plan, Cost of Providing Special and Contractual Termination Benefits 1 0 $ 0
Net transfers from variable fund/401(k) plan 0 0  
Expenses paid 0 0  
Benefits paid 36 34  
Actuarial Gain (Loss) from Changes in Discount Rate 24 43  
Actuarial Gain (Loss) from Delaying post-Medicare Trend Rate Assumption 28    
Change in plan assets      
Fair value of net plan assets 0 0  
Actual return on plan assets 0 0  
Employer contributions 24 22  
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]      
Funded status $ (291) $ (353)  
Discount rate 5.62% 5.00%  
Amount of defined benefit plan actuarial gain (loss) from updated capita claims costs and retiree contributions $ (3)    
[1] Collections include retiree contributions as well as provider discounts and rebates.
(2) The 2025 pension obligation plan curtailment gain is a result of the amendments to the TVA SERP in which participants ceased accruing new benefits effective September 30, 2025. This reduced the projected benefit obligation by $1 million.
(3) Special/contractual termination benefits for certain eligible employees related to TVA's restructuring activities. For 2025, the other post-retirement plan recognized a loss that increased the obligation by $1 million as a result of special/contractual termination benefits for certain eligible employees related to TVA’s restructuring activities. See Note 3 — Restructuring.